My players need this for the up and coming campaign, for some reason I didn't put it into the book in time!

All non-gas planets are classes by how inhabitable they are. The higher the letter, the more inhabitable they are.

A - Volatile tectonics, toxic atmosphere, hot (proximity to the star). Not possible to live under, on or above the surface.B - Volatile tectonics, toxic atmosphere, cool (not close to the star). Possible to live above the surface on a giant grav platform.C - Highly toxic atmosphere, hot. Comfortable to live under the surface. On surface possible but not recommended.D - Highly toxic atmosphere, cool. Comfortable to live under the surface. On the surface possible.E - Low toxicity atmosphere, not breathable but not dangerous to skin contact. On or below the surface equally comfortable.F - Low toxicity atmosphere, barely breathable but not more than a day at a time. On or below the surface equally comfortable.G - Low toxicity atmosphere. Long term breathing might lead to death but not immediately lethal. Candidate for atmosphere generation to turn into H or I class.H - Earth-like stable atmosphere but only through constant atmosphere generation.I - Earth-like world, does not require atmosphere generation (although may have needed it to get to this state)

That's right! That's where I break from realism and deal with only those planets that have gravity similar to Earth. I looked into adding that but it didn't add much to the game. There are more than enough options to get a GM/player started. If pushed really hard, I would argue that Earth-like-size planets are common. The C21st hasn't found many but they ARE common, OK? THEY ARE COMMON! GET IT? COMMON.

If I was creating a harder Sci Fi then I would definitely include gravity/radiation etc. Not only does it add flavour to the locations but it gives that feel of the unknown and strange. For Icar, it doesn't add a great deal and would bloat a game that's already quite overweight!